This invention relates to fuel cells aid, more particularly, to tubular solid oxide fuel cell assemblies and fuel cell devices incorporating such assemblies.
A fuel cell is an electrical device which converts the energy potential of fuel to electricity through an electrochemical reaction. In general, a fuel cell unit comprises a pair of electrodes (anode and cathode) separated by an electrolyte. The electrolyte permits only the passage of certain types of ions. The selective passage of ions across the electrolyte results in a potential being generated between the two electrodes. This potential can be harnessed to do useful work. This direct conversion process increases the efficiency of power generation by removing mechanical steps required by traditional power generating devices such as internal combustion engine-driven electrical generators. Additionally, the combination of higher efficiency and electrochemical processes provides an energy-efficient, resource-conserving and environmentally sound source of electrical power.
A well known type of solid oxide fuel cell (SOFC) possesses three basic components: an anode layer that generates electrons, a cathode layer that consumes electrons and an intermediate electrolyte layer that conducts ions but prevents electrons from passing. In operation, a fuel such as hydrogen, as hydrocarbon, an alcohol, carbon monoxide or mixture of any of the foregoing combines with oxygen anions within the anode layer to produce water and/or carbon dioxide and electrons. The electrons generated within the anode layer migrate through the external load and back to the cathode layer where oxygen, typically provided as a flow of air, combines with the electrons to provide oxygen anions which selectively pass through the electrolyte layer and into the anode layer.
There are in general three structural types of SOFCs: monolithic SOFCs which possess a honeycomb construction formed by fusing together thin sheets of ceramic material into a monolithic block; tubular SOFCs which as their name indicates possess a tubular, typically cylindral, configuration; and, planar SOFCs which possess a flat, plate-like shape, SOFCs operate at fairly high temperatures, e.g., on the order of 850-1000° C. As a result of these high operating temperatures, planar SOFCs have a tendency to crack and to encounter problems with sealing resulting from thermal cycling. While tubular SOFCs generally perform better in these respects, they may exhibit operational difficulties relating to current collection such as ohmic losses resulting from separation of electrical contact surfaces occurring during operation. Monolithic SOFCs, owing to the large number of small components, layers and interconnections involved in their manufacture, pose heightened concerns for their reliability.
In the particular case of tubular SOFC assemblies and fuel cell devices incorporating them, thermal stresses resulting from on-off cycling can lead to ohmic losses due to the tendency of their current collector components to pull away or separate from the electrodes with which they are associated under operating conditions. The subsequent reduction in the area of electrical contact is due largely to the difference in thermal expansion of the ceramic electrode components of the tubular SOFC assemblies compared with those of their metal or metal-containing current collector components. Mechanical threes resulting from the differential in thermal expansion of the electrodes and current collectors, although individually small, may exert a cumulative effect over time manifested as a permanent significant reduction in area of electrical contact between current collectors and ceramic electrodes and accompanying power-robbing ohmic losses.
A need therefore exists for a tubular SOFC assembly that resists the aforenoted tendency for the current collector components of the assembly to pull away or separate from their associated electrodes during operation.